The present disclosure relates generally to non-invasive blood pressure monitoring and, more particularly, to a method and apparatus for determining blood pressure using pressure pulse duty cycle.
Since 1733, when the Reverend Stephen Hales first inserted a long, upright glass tube into the artery of a horse, the direct and invasive measurement of blood pressure remains the most accurate method for obtaining a blood pressure measurement. To obtain such accurate measurements, a catheter is inserted surgically a patient""s artery and guided to the measurement site. However, due to the risk of infection and blood loss (as well as for patient comfort and convenience), non-invasive measurements are most commonly used.
Of the various non-invasive blood pressure measurement techniques in existence today, the most common is the auscultatory technique, based on the ability of the human ear to detect and distinguish sounds. In 1905, Korotkoff first described these auscultatory (i.e., characteristic) sounds that became the foundation for the auscultatory technique. In this technique, an air-filled cuff is wrapped around a patient""s upper arm and is then inflated to occlude the brachial artery. As the cuff is allowed to deflate, a stethoscope is placed over the patient""s brachial artery (distal to the cuff). The clinician uses the stethoscope to listen for the Korotkoff sounds as the cuff deflates. Upon gradual release of the constricted pressure, the beginning of blood flow may be heard. At that time, the pressure reading on a gauge (in millimeters of mercury) is noted, and is referred to as systolic pressure. The pressure is then further released until the sounds of flow again cease, at which time the pressure reading is once again noted. This reading is referred to as diastolic pressure.
One disadvantage of the sound dependent, auscultatory technique stems from the fact that some patients may exhibit muted sounds as a result of a condition such as hypotension (low blood pressure), for example, thus making the measurement difficult to detect. In addition, there is the possibility of measurement error due to differences in hearing acuity and measurement technique from clinician to clinician. Further, unqualified or inexperienced personnel may be more susceptible to outside noise, interference, or an inconsistent assessment of Korotkoff sounds. In an attempt to increase reproducibility, some automated devices have even replaced the human ear with a microphone.
Another existing non-invasive blood pressure measurement technique is the xe2x80x9coscillometricxe2x80x9d method, which refers to any measurement of small pressure oscillations in an occlusive cuff caused by the arterial pressure pulse. The oscillometric method, like the auscultatory method, uses an occlusive cuff applied around the upper arm. However, instead of using microphones or stethoscopes to listen for characteristic sounds, oscillometric devices use pressure signal pulses picked up by the cuff to determine blood pressure. These pressure signal pulses are transmitted to the cuff by the pulsatile blood flow through the brachial artery. The signals are analyzed to determine a measure mean arterial pressure (MAP), identified as the point of maximum pulse amplitude. From this measurement, systolic and diastolic blood pressures are estimated.
Fundamentally, the amplitude approach to identifying MAP and estimating systolic and diastolic pressures appeals to the nonlinear relationship between the artery cross sectional area and the transmural pressure (the difference in pressures inside and outside the artery), primarily due to the material properties of the artery wall.
However, there are several other factors that can modify the amplitude of the pulse detected by the monitoring device including, but not limited to, changes in arterial compliance, material properties of other tissues in the arm, cuff material and wrap, and pneumatic system characteristics. Accordingly, it is desirable to be able to implement an alternative method of oscillometric measurement that avoids the complexities of measuring pulse amplitudes.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a method of processing oscillometric blood pressure pulse data taken from a subject. In an exemplary embodiment, the method includes determining a pressure pulse period of a cardiac cycle of the subject and identifying a duration of a pressure pulse detected within the pressure pulse period. Then, a duty cycle of the pressure pulse is calculated with respect to the pressure pulse period, wherein the calculated duty cycle is used to determine selected blood pressure parameters.
In another aspect, a method of measuring the blood pressure of a subject includes inflating a cuff placed over an arm of the subject, thereby applying a plurality of pressure levels thereto. At each pressure level, obtaining pulse pressure data caused by blood pressure pulses within the arm of the subject is obtained and, from the pulse pressure data, a duty cycle for a pressure pulse period is calculated. The duty cycle is then used to determine selected blood pressure parameters.
In still a further aspect, an apparatus for obtaining the blood pressure of a subject includes an inflatable and deflatable pressure cuff for placement around an arm of the subject. An inflating apparatus is used for inflating and pressurizing said pressure cuff, while a deflating apparatus selectively relieves pressure from the cuff. In addition, a pressure sensing device is coupled to the cuff for sensing pulse pressure data caused by blood pressure pulses within the arm of the subject. A microprocessor processes the pulse pressure data by determining duty cycle information for a pressure pulse period at a given cuff pressure level. The duty cycle information is used to determine selected blood pressure parameters.
In still another aspect, a storage medium includes a machine readable computer program code for processing oscillometric blood pressure pulse data taken from a subject, as well as instructions for causing a computer to implement a method. The method further includes determining a pressure pulse period of a cardiac cycle of the subject, and identifying a duration of a pressure pulse detected within the pressure pulse period. A duty cycle of the pressure pulse is calculated with respect to the pressure pulse period, wherein the calculated duty cycle is used to determine selected blood pressure parameters.
In still another aspect, a computer data signal includes code configured to cause a processor to implement a method for processing oscillometric blood pressure pulse data taken from a subject. The method further includes determining a pressure pulse period of a cardiac cycle of the subject, and identifying a duration of a pressure pulse detected within the pressure pulse period. A duty cycle of the pressure pulse is calculated with respect to the pressure pulse period, wherein the calculated duty cycle is used to determine selected blood pressure parameters.